In holographic storage, data is stored in a hologram resulting from the interference of a signal and reference beam. During storage, both the reference and signal beams are incident on the storage medium. During retrieval, only the reference beam is incident on the medium. The reference beam interacts with the stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram. Relative to conventional magnetic and optical data storage methods, holographic data storage promises high storage densities, short access times, and fast data transfer rates. The widespread use of holographic data storage has been hindered by the relative complexity of the specialized components required for storage and retrieval of data.
For information on conventional volume holographic storage see for example U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669. In conventional volume holographic storage, each bit is stored as a hologram extending over the entire volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages are stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as 10.sup.6 bits within one page can be stored and retrieved simultaneously. Conventional volume holographic storage generally requires complex, specialized components such as amplitude and/or phase spatial light modulators, however.
In U.S. Pat. No. 4,458,345, Bjorklund et al. describe a bit-wise volume holographic storage method using signal and reference beams incident on a rotating disk in a transmission geometry. The signal and reference beams are incident from the same side of the disk. The angle between the reference and signal beams can be altered to store holograms at various depths within the medium. A separate photodetector is used to retrieve data stored at each depth. The interaction of light with the medium is localized through two-photon recording.
In U.S. Pat. No. 5,659,536, Maillot et al. describe a system in which multiple holograms are stored at each location in a disk through wavelength multiplexing. Each hologram spans the depth of the medium. In U.S. Pat. No. 5,289,407, Strickler et al. describe a multi-layered, non-holographic, index-perturbation optical storage system. Bits are stored as localized perturbations in the index of refraction of a photopolymer, caused by the high intensity at the focus of a single laser beam.
Holographic recording of data on a tape medium is also described. Examples are found in U.S. Pat. No. 5,519,517 by Redfield et al., U.S. Pat. No. 4,135,251 by Ruell, U.S. Pat. No. 4,045,115 by Lee and U.S. Pat. No. 3,947,640 by Ruell et al.